Support Argon2

[dhui]

Argon2 is now available in the golang.org/x/crypto/argon2 package. https://godoc.org/golang.org/x/crypto/argon2

Note: As of 2017-01-07, the Golang implementation chose to only expose Argon2i which is optimized for password hashing and KDFs since it's data independent. See:

• https://tools.ietf.org/html/draft-irtf-cfrg-argon2-03#section-1
• https://tools.ietf.org/html/draft-irtf-cfrg-argon2-03#section-4 (recommends Argon2id for general usage)

Hold off on making Argon2 the default KDF until more time has passed: https://pthree.org/2016/06/29/further-investigation-into-scrypt-and-argon2-password-hashing/

[dhui]

golang.org/x/crypt/argon2 now "supports" the Argon2id variant by exposing it:
golang/go#23602
golang/crypto@d9133f5